Building on a slope plot can be a complicated matter, making the project more complex and possibly significantly adding to the cost. Here are shown the various implications of building on a slope.
The main Implications of Building on a Slope
The cost of building on a slope is increased by various factors. A significant factor is the additional materials necessary, such as reinforced suspended concrete floors, most commonly pre-cast beams. Other things to consider are hard landscaping, the extra drainage required (perhaps including the necessity of a pump), and any extra work needed in stepping the foundations of the building.
Costs are always individual to each build, and so it is not perfectly safe to try to determine the cost of a build based purely on a set of certain factors. The contributing factors to cost are almost always variable. Average build costs can safely be relied upon until the point of completing the detailed design drawings, but once work has commenced it is best to cost each project upon only its own merits.
Cut and Fill
Cutting and filling is the process of carving out a level area on a sloping surface so as to allow the building of a normal house designed to be built on a level surface. What is cut from the slope is retained so that it may be used to build up the levels on the lower edge. Building on a slope will always increase the costs of the foundations, especially considering that the foundations should find bearing on an original subsoil. Where the spoil is piled against the built up or the lower section of the house then it must be designed so as to ensure that the building is built up to within 600mm of the proposed soil level on the 0utside. The is so that the pressure on the walls in equalised. However, it is expensive to haul the spoil from the building site and can be particularly time consuming. Retaining the spoil dug from the slope can be beneficial in both time and cost management, just as long as there is space enough to keep it stored until it is needed.
Drainage and Sewerage
The extra costs of building on a slope may also be added to when it comes to drainage and sewers. In cases where the site slopes downwards from a road in which the mains sewers are installed fairly shallowly it might be necessary to install a pumped sewage system to get the sewage up the system. This can cost at least an extra £2,000, but you may find that you benefit from similar savings due to the much cheaper cost of the 50mm flexible pipe than that of a conventional drainage pipe.
If your site is situated down from the road but the sewers are set deeply beneath the road, you may actually benefit from your location in terms of sewerage and drainage. This is because the house drainage will not have to be as deep. A site which is located up a slope from the road may logically be more practical in terms of drainage and sewerage, but very steep slopes might necessitate the installation of tumble bays inside the manholes. This slows off the fall of the sewerage, so that it does not enter the sewers too quickly.
Another consideration when building on a slope is rainwater. It may at first seem best to be situated up from the road, but many local authorities do not allow surface water into their sewers and many insist upon it not being allowed to reach the road either. However, being situated down from the road may mean that water collects on the lower floor of the house or garage. This can be helped by adding a drainage channel to divert the rainwater away from the house.
Find out about off-mains drainage here.
Basements are usually no less expensive, and often more expensive, than the rest of the home to build. A basement may actually be completely impossible to build if building on ground with with high water tables or in heavy clay. However, it may be the case that the land you are building on requires a basement, either full or partial, for the sake of the structure’s safety.
The basement walls of any house, but particularly any building on a slope, must be sufficiently strong to deal with the pressure of the earth around them. These walls are basically retaining walls in their own right. However, when building on a level area beside a bank you may find that separate retaining walls are required. Retaining walls of over 1,200mm in height must be designed by an engineer, and in some cases it could be cheaper to create a number of lower retaining walls with stepped ground between each. An alternative – wire cages known as Gabions – filled with stone or concrete, and then filled with soil and planted in the ground, can also be used.
Building on a Slope with Stilts
One interesting way of building on a slope is to construct the dwelling on stilts or columns. This avoids the need for additional foundations and tanking. This method also means the ground is not needed to be tampered with, allowing for planting over of much of the site. Stilts or columns can be used for multi-storey properties and can in fact be a very cost-effective option.